Method for producing a thin film

Abstract: A method of producing thin-film products consisting of injecting the molten resin into a die. The internal cavity pressure is momentarily reduced when the internal cavity pressure reaches a prescribed pressure, and closing of the cavity is performed so that the final thickness of the molded product is achieved, and a thin-film molded product produced is produced by said method.

Description: The present invention relates to a method of producing thin films of synthetic resin, for example, a molded product with a film thickness of less than approximately 1 mm. Synthetic resins, especially, various types of molded products consisting of materials comprised of engineering plastics, also referred to as high performance plastics, are widely used for many purposes. For example, they are widely used for housings and chassis for small portable machines of notebook or lap-top style. A weight that is as light as possible is required for housings for said types of small machines. The easiest method to achieve weight reduction in molded resin products is to reduce the thickness of the film. However, in order to produce a thin-film molded product with a thickness of less than 1 mm, for example, it is necessary to perform the injection at a high pressure so that the molten resin injected from the injection cylinder flows within a narrow cavity at a high speed. When the molten material is injected from the nozzle at a high injection pressure, the pressure of the molten material in the cavity is drastically increased, and as a result the die members tend to part. When this happens, it is difficult to produce a molded product with the desired thin-film thickness and design. It is not practical to provide a closing force greater than a predetermined value to eliminate the above-mentioned problems from the standpoint of mechanical structure and cost factors. Therefore, application of compression injection molding, in which the gap of the die cavity is set at a value greater than the prescribed thickness, and closing is done while the molten material is being injected to restore the prescribed cavity gap, is conceivable. As an example, the compression injection molding method disclosed in Japanese Kakai Patent Application No. SHO 60[1985]-110419 (applicant, Idemitsu Petrochemicals, Ltd.), etc. can be mentioned. However, in a high performance molding that achieves adequate physical properties even when the thickness of the film is low, that is, upon molding of engineering plastics, closing is inhibited due to the internal cavity pressure generated at the final filling stage, especially at the time of production of a thin-film molded product with a film thickness less than 1 mm, and a greater closing force is required; as a result, it is difficult to produce a film with the prescribed thickness. BE-A-559392 describes a process for manufacturing moulded articles in accordance with the preamble of claim 1. Patent Abstract of Japan vol.10 no.21 (M-449) & JP-A-60179216 shows an injection moulding process in which the state of fill of the mould is measured, for example, by measuring the pressure in the mould, The object of the present invention is to eliminate the above-mentioned problems with the conventional methods, and to provide a method of producing a thin film capable of producing a molded product from engineering plastics having a uniform thickness. According to the invention there is provided a method of producing a thin film of plastics material according to the steps recited in claim 1. Preferred embodiments are defined in claims 2 to 4. Closing of the die and reduction of the pressure may take place when the internal cavity pressure near the gate has reached 200-300 kg/cm 2 . The pressure sensor may be installed at 70-80% of the flow distance of the molded product. When the internal cavity pressure detected by the sensor reaches 200 kg/cm 2 to 300 kg/cm 2 , which is approximately half of the standard value of the internal cavity pressure during standard injection molding, the internal cavity pressure is momentarily reduced by withdrawal of the screw. The internal cavity pressure is detected by a pressure sensor installed at approximately 70-80% of the flow distance of the standard filling. As a result of the pressure drop described above, the resin in the inner core layer that is capable of flowing undergoes a backflow, and the undesirable increase in internal cavity pressure can be prevented. As a result, excessive closing force is not required at the final stage of the closing that forms the desired film thickness. Therefore, according to the method of producing the think film of the present invention, excessive closing force is not required, and a think film molded product with an accurate film thickness can be produced. The invention will now be described by way of example with reference to the accompanying drawings in which:- FIGURE 1 and FIGURE 2 are model diagrams of dies used for practising the method of producing a thin film of the present invention; FIGURE 3a shows an example of the molded product produced by the method of the present invention; FIGURE 3b is a cross-section along the line A-A of Figure 3a; and FIGURE 4 is a time chart that exhibits a change in each area in the method of producing the thin film of the present invention. In Figure 1, molten material 6 is injected from the tip of nozzle 5 of the injection cylinder 4 into the cavity 3 formed between stationary die 1 and movable die 2 to produce the desired molded product. In the injection cylinder 4, a common pressure unit 7, such as a screw or plunger is used. In this case, in Figure 1, the pressure is applied in the direction I indicated by the arrow, and injection of the molten material 6 is accomplished. then, closing is performed by moving the movable die 2 in direction II indicated by the arrows in Figure 2, and the desired thickness of the molded thin-film product can be achieved. As shown in Figures 3a and 3b, the molded product is a box-form molded product with a 250 x 180 rectangle base with a thickness of 0.8 mm, and injection of the molten material was accomplished through hot runner gates 10 at 4 points. According to the time chart shown in Figure 4, in the method of producing the thin film of the present invention, the thermoplastic resin molten material in the injection molding machine, which is not shown in the figure, is injected into the die in which the space in the cavity has been expanded ahead of time (injection starting time a). Subsequently, the hydraulic pressure used for applying pressure to screw 7 in the injection machine ramps up to the set value, but the internal cavity pressure does not increase suddenly since the molten resin 6 flows inside the resin. Before the prescribed amount of the molten material 6 is filled inside the cavity, the moveable die 2 is moved forward in the direction II of the arrows, and the process referred to as closing (start of closing b) is performed, and the cavity gap is decreased to the prescribed thickness. For this reason, the cavity pressure in the die gradually increases, and at the time when the internal cavity pressure increases to approximately 200-300 kg/cm 2 , supply of the hydraulic pressure to the injection cylinder is terminated, and hydraulic pressure is supplied in the opposite direction to move the screw in direction IV (injection completion c). As a result, the closing force against the die overcomes the pressure within the cavity, and excess molten material within the cavity flows back toward the injection cylinder through the gates. Closing is further continued, and the die is completely closed (closing completed d), and the pressurized injection process is achieved, For this reason, the internal cavity pressure gradually decreases along with shrinkage of the resin associated with the decrease in temperature, and eventually it becomes zero. For the method described, it is desirable to use a die having an open type hot chip, in which temperature control, especially around the gate area, is possible. When said die is used, a backflow of the molten material from the gate toward the injection cylinder can be achieved smoothly, and in this application example, a thin-film molded product with a uniform thickness of 0.8 mm was achieved despite having a film thickness of less than 1 mm. According to the method of producing the think film of the present invention, in order to prevent a sudden increase in the internal cavity pressure during the final stage of closing, excess molten material, which is capable of flowing at this stage, is permitted to flow back toward the injection cylinder through the gates under the conditions described above. As a result, excess closing force is not required and a thin-film molded product with a relatively large size can be produced in a molding machine with a relatively low closing force. By means of the method of producing the thin-film of the present invention, it is possible to produce a uniform molded product with a thin-film thickness of 1 mm when the cavity in the die is decreased while injecting the molten material into the cavity in a pressurized injection-molding system.

Claims: A method of producing a thin film of plastics material having a desired thickness, comprising the steps of injecting molten resin into a cavity formed between a stationary die and a movable die with the movable die positioned so that the cavity is opened wider than the desired thickness of the thin film; and closing the dies by relative movement of the dies towards each other so that the cavity reaches the desired thickness; characterized by starting the die closing operation while injection is proceding, monitoring the pressure within the cavity by means of a pressure sensor as the injection step proceeds, and discontinuing injection of resin when a predetermined pressure has been reached to allow backflow of the molten resin from the cavity during the closing step. The method of claim 1, wherein the predetermined pressure is between 200 and 300 kg/cm 2 . The method of claim 1, wherein the desired thickness of the film is less than 1mm. The method of claim 1 or 2 wherein the pressure sensor is installed at 70-80% of the flow distance of the molded product.

Inventors: KUBOTA ATSUSHI, KUBOTA, ATSUSHI
Applicants: GE PLASTICS JAPAN LTD, GE PLASTICS JAPAN LIMITED
Application year: 1994
Main classification: B29C45
IPC codes: B29C45